A photorelay, which is an example of an optical coupling device, is sometimes used as a signal selector switch in measuring instruments such as a tester. In this case, the total installation area of the photorelays is increased since a large number of photorelays is used. Therefore, in such applications, photorelays of a small size are preferable.
Examples of a photorelay used as the signal selector switch include a photorelay that switches between input states in which a high-frequency signal is input or not to an object to be measured and a photorelay that switches between states in which a low-frequency DC signal is input or not to the object to be measured. The output terminals of these different photorelays are electrically connected to each other by external wiring. As a result, there is a possibility that, when the high-frequency signal is being input via one photorelay in an ON state a current leaks via the external wiring to the another photorelay that is in an OFF state. This current leakage affects the characteristics of the other photorelay.